As part of a longitudinal, collaborative behavior genetics study in Colorado, behavior problems were studied in 5 year-old monozygotic and dizygotic twins. The roles of genetic and shared environmental factors were assessed, based on multiple informants (mother, father, teacher). Genetic influence on externalizing problems, internalizing problems, and attentional/activity problems was identified across measurement contexts. Effects of the shared environment were identified as well but with less consistency (i.e. only from parents, not teachers, and more from father than mother reports). Correlations between observed patterns of regulation at ages 3, 4, and 5 years and behavior problems at 5 years, were often significant, though low in magnitude, for teacher reports of child problems. Adaptive internalization of control at ages 3, 4, and 5 (e.g. frustration tolerance, capacity to attend and focus, good impulse control) predicted fewer externalizing and attentional problems at age five. Internalization of standards, reflected in children's understanding of reciprocity and social problem solving also were sometimes associated with fewer problems. Boys were consistently reported to have externalizing problems (aggression, hostility, attentional problems and hyperactivity) than girls, but there were no sex differences for internalizing problems (anxiety, depressed mood). Correlations between internalizing and externalizing problems were high, indicating significant co-morbidity of problems in young children. Social maturity and emotion regulation at ages 3 and 4 was more characteristic of girls than boys, suggesting early protective factors that may mitigate against the development of behavior problems in females during childhood.